The present invention relates to a telephone subscriber circuit of a telephone exchange comprising active circuit elements with a capacitive, complex input impedance, wherein the signal which is to be transmitted from the exchange to the subscriber is amplified by two operational amplifiers operating in push-pull and is fed via two coupling arms, formed by a series arrangement of a capacitor and a resistor, into the subscriber loop which is supplied wit d.c. current via two supply resistors, whilst the symmetrical subscriber signal is supplied to a subscriber amplifier via two arms of the compensation network, which serves to directionally isolate the signals of the two speech directions.
In an endeavor to improve the side-tone reference attenuation at the subscriber station and thus the transmission quality, efforts to improve the electrical matching of the subscriber circuit to the subscriber speech stations are of particular importance. In conventional telephone systems an input impedance of 600 Ohms was provided in real form, but did not comply with the high transmission quality requirements.
An aim of the present invention is to provide a telephone subscriber circuit which complies with the stated requirements, which it achieves in that the entire complex input impedance is subdivided into a complex component which, as coupling arm, serves to supply the LF-signal which is to be transmitted and to block the d.c. current from the operational amplifiers, and into a real component via which the supply d.c. current is supplied.
By means of this circuit in accordance with the invention, a determinate, capacitively-complex input impedance of the subscriber circuit can be achieved which facilitates the qualitatively high grade matching. A sufficiently powerful supply d.c. current can be used without the need for an unreasonably high supply voltage.
The capacitors required in the coupling arms in order to produce the complex input impedance simultaneously serve to block the d.c. current of the amplifiers from the speech wires, without the need for high capacitance values for this purpose.
In accordance with a further feature of the invention, the high-frequency emphasis produced by the coupling arms is counteracted by a frequency-dependent negative feedback of the amplifier.
A further feature of the invention consists in that disturbance of the input impedance by the compensation network is avoided by the high impedance design of the letter.
The requirement that the input impedance and balancing impedance should be identical facilitates a transformation of the compensation network which leads to a simple construction of the network requiring few components. The arms of the compensation network contain only resistors, so that the reception of the subscriber signal is governed by a linear frequency response. In order to fulfill the symmetry requirement, with the selected sub-division of the input impedance (into supply resistor and coupling arm), and with the described transformation of the compensation network, it is merely necessary that four capacitors should match one another in pairs.